1. Technical Field of the Invention
The present invention relates to an apparatus for driving electromagnetic valves, and in particular, to the apparatus adapted to discharge high-voltage energy charged to a discharging capacitor, into a coil in an electromagnetic valve to enhance the operational responsiveness of the electromagnetic valve.
2. Related Art
Fuel injection valves are used, for example, for injecting and supplying fuel to individual cylinders of an internal combustion engine loaded on a vehicle. Electromagnetic valves have been used as fuel injection valves, which are opened by supplying current to coils. Such fuel injection valves are driven and controlled by a fuel injection control apparatus. In particular, a fuel injection apparatus controls a current supply period and current supply timing for each of the coils, so that the amount and timing of fuel injection can be controlled for the internal combustion engine.
A type of such a fuel injection control apparatus is known as disclosed in Japanese Patent Application Laid-Open No. 2001-015332. In this apparatus, a power source voltage is raised by a booster circuit to charge a discharging capacitor. At the beginning of a driving period for supplying current to the coils, the high-voltage electrical energy stored in the discharging capacitor is charged to the coils of fuel injection valves to pass a predetermined high current (so-called peak current) therethrough. In this way, the fuel injection valves are promptly brought into an open state. Then, a current of a certain level is passed through the coils until the driving period expires, keeping the fuel injection valves in an open state.
With reference to FIG. 12, an example of a particular configuration of this type of fuel injection control apparatus will be explained.
A fuel injection control apparatus 100 of FIG. 12 is purposed to drive electromagnetic solenoid type unit injectors (hereinafter just referred to as “electromagnetic valve(s)”) for injecting and supplying fuel to, for example, cylinders of a multi-cylinder diesel engine loaded on a vehicle. FIG. 12 shows only one of the electromagnetic valves for the respective cylinders. The following explanation is focused on the driving of a single electromagnetic valve 21 shown in FIG. 12. In this example, transistors used as switching elements are MOSFETs, which hereinafter are referred to as “switching elements”.
The electromagnetic valve 21 is a normally-closed type electromagnetic valve provided with a coil 21a. When current is supplied to the coil 21a, a valve body, not shown, moves against the bias force of a return spring to a valve-opening position so that the fuel is injected. When the current supply to the coils 21a is finished, the valve body returns to a valve-closing position, or an original position, so that the fuel injection is stopped.
The fuel injection control apparatus 100 includes: an output terminal P1, to which high(Hi)-side terminal of the coil 21a of the electromagnetic valve 21 is connected; an output terminal P2, to which a low(Low)-side terminal of the coil 21a is connected; a switch 13 for driving the electromagnetic valve, which is serially connected between an end of a current detection resistor R1 and the output terminal P2, the other end of the resistor R1 being connected to a ground line (GND=0V); a switch 12 whose one output terminal is connected to a power source line, to which voltage (battery voltage) VB of an on-vehicle battery is supplied as power source voltage; a diode D2 for preventing back flow, whose anode is connected to the other output terminal of the switch 12 and whose cathode is connected to the output terminal P1; a capacitor (discharging capacitor) C1 for passing peak current to the coil 21a, the peak current serving for promptly bringing the electromagnetic valve 21 into an open state; a DCDC converter 23 for boosting the battery voltage VB to generate voltage higher than the battery voltage VB, so that the capacitor C1 can be charged by being supplied with the high voltage through a diode D4; a switch 11 for connecting a positive terminal (opposite to the ground line side) of the capacitor C1 to the output terminal P1 (and further to the upstream-side terminal of the coil 21a); a diode D3 for a flywheel (circulation), whose anode is connected to the ground line and whose cathode is connected to the output terminal P1; and a control circuit 25 made up of a microcomputer, for example, for controlling the switches 11 to 13 and the DCDC converter 23.
The fuel injection control apparatus 100 also includes: an energy recovery path 22, which is provided between the output terminal P2 and the positive terminal of the capacitor C1 to recover flyback energy circulated from the downstream-side of the coil 21a to the capacitor C1; and a diode D1, which is provided upstream of the energy recovery path 22 with its cathode being directed to the capacitor C1 to control the direction of current.
Practically, the output terminal P1 is commonly used between a plurality of electromagnetic valves for the respective cylinders, i.e. coils of the individual electromagnetic valves are connected to this single output terminal P1. The output terminal P2 and the switch 13 are provided for the coil of every electromagnetic valve.
The DCDC converter 23 includes an inductor and a switch, which are arranged in series between the power source line of the battery voltage VB and the ground line. It is known that the capacitor C1 is charged by the flyback voltage through the diode D4. The flyback voltage is generated in the inductor by the turning on/off of the switch.
Operation of the fuel injection control apparatus 100 configured as described above will be described with reference to a timing diagram of FIG. 13.
The control circuit 25 determines a driving period for supplying current to a coil of an electromagnetic valve for every cylinder, based on engine operation information, such as engine speed and accelerator opening, and turns on the switch 13 corresponding to the electromagnetic valve in question only during the driving period.
Prior to the commencement of the driving period of each electromagnetic valve, the control circuit 25 operates the DCDC converter 23 to charge the capacitor C1 up until the charge voltage (positive terminal voltage) of the capacitor reaches a target voltage Vc1.
As shown in FIG. 13, when time comes for starting the driving period for any one of the electromagnetic valves, the control circuit 25 turns on the switch 13 corresponding to the electromagnetic valve in question, while also turning on the switch 11. In the example shown in FIG. 13, the switch 12 is also turned on together with the switch 11.
Then, an electrical connection is established between the positive terminal of the capacitor C1 and the output terminal P1 through the switch 11, so that the energy charged to the capacitor C1 is discharged to the coil 21a, whereby current supply is started for the coil 21a. The discharge of the capacitor C1 at this moment permits high current (peak current) to flow through the coil 21a, for promptly bringing the electromagnetic valve 21 into an open state.
In FIG. 13, the “Current at output terminal P1” indicated by a broken line is coil current that flows through the coil 21a. During the discharge from the capacitor C1, the diode D2 prevents the battery voltage VB from sneaking out of the output terminal P1, whose potential has been raised, to the power source line. Further, even when the switch 13 is turned on, the diode D1 will prevent current from directly flowing into the switch 13 from the positive terminal of the capacitor C1 through the energy recovery path 22.
The control circuit 25 turns on the switch 11 and, after lapse of a certain period, turns off the switch 11. It should be appreciated that another configuration may be used, in which coil current is detected in terms of voltage generated at the resistor R1, and when the coil current reaches a target current, or a peak current, the switch 11 is turned off.
Thus, at the beginning of the driving period, the switch 11 is turned on to permit the capacitor C1 to discharge current to the coil 21a, whereby high current flows through the coil 21a to accelerate valve-opening response of the electromagnetic valve 21.
While the switch 11 is turned on, the control circuit 25 inhibits the DCDC converter 23 from charging current to the capacitor C1 in order to stabilize the discharge current of the capacitor C1.
After turning off the switch 11, the control circuit 25 performs on/off control of the switch 12, so that the coil current detected in terms of voltage generated at the resistor R1 will be kept at a constant level, which is smaller than the target current, or the peak current.
Once the switch 11 has been turned off, such constant current control of the switch 12 allows a constant current to flow from the power source line of the battery voltage VB to the coil 21a through the switch 12 and the diode D2, which constant current keeps the electromagnetic valve 21 in an open state.
In case the switch 11 or 12 has been turned off in the state where the switch 13 is turned on, flyback current is circulated from the side of the ground line to the coil 21a through the diode D3. Accordingly, current that passes through the coil 21a is the flyback current circulated through the diode D3 during the period from when the switch 11 is turned off to when the on/off control of the switch 12 is started, and during the period when the switch 12 is turned off in the on/off control of the switch 12.
When the driving period has elapsed, the control circuit 25 turns off the switch 13 and terminates the on/off control (i.e., constant current control) of the switch 12 to keep the switch 12 in an off-state. Thus, current supply to the coil 21a is stopped to close the electromagnetic valve 21. As a result, the fuel injection performed by the electromagnetic valve 21 is ended.
The flyback energy produced at the coil 21a when the switches 13 and 12 are turned off is recovered in the form of current from the downstream side of the coil 21a to the capacitor C1 through the diode D1 on the energy recovery path 22.
On the other hand, after turning off the switch 11 or 13, the control circuit 25 allows the DCDC converter 23 to resume charging the capacitor C1. This is for preparing for the subsequent driving of the electromagnetic valve. In the example shown in FIG. 13, while the switch 11 is turned on, the switch 12 is also turned on. Alternatively, the switch 12 may be adapted to turn off when the switch 11 is turned on. The configuration and operation of a fuel injection control apparatus of the type as described above are also described in detail in Japanese Patent Application Laid-Open No. 2001-015332, for example.
When the switch 11 is switched from an on-state to an off-state in the conventional fuel injection control apparatus 100 described above, current is supplied from the ground line side to the upstream-side terminal of the coil 21a through the diode D3.
Thus, when a forward-direction voltage at the diode D3 is “Vf”, the voltage at the output terminal P1 in the state where the switch 11 is turned off drops at once from the charge voltage (substantially the target charge voltage Vc1) of the capacitor C1 to “−Vf” as shown in a broken-line square in FIG. 13, allowing a voltage variation ΔV per unit time to become very large. As a result, emission noise of an amount corresponding to the voltage variation may be caused to adversely effect reception conditions, for example, of radios.
One approach for suppressing the emission noise may, for example, be to reduce the electrical energy (i.e., voltage and current per se) fed to the coil 21a. Another approach may be to increase a slew rate of the switch 11 (i.e., to gradually bring the switch 11 into an off-state). The former approach however may deteriorate valve-opening responsiveness of the electromagnetic valve 21. The latter approach is also not practical because it may increase power loss of the switch 11, which may also increase the heat generation rate. Therefore, it has been desired that heat generation of elements be reduced as much as possible in on-vehicle apparatuses in particular, which are placed under high-temperature conditions.